This invention relates generally to heat treatment of serviced turbine parts and, more particularly, to a method for extending the useful life of a serviced turbine part.
Gas turbine engines operate at high temperatures and high stresses. Gas turbine components or parts, including, without limitation, rotor disks having wheels and spacers, are subjected to high temperatures and high stresses. The turbine parts are frequently inspected to ensure that cracking is not present in the parts. Cracking may result from one or more mechanisms, such as hold time fatigue cracking. Hold time fatigue cracking occurs as a result of extended in-service hours at high temperatures and/or high stresses and is common among nickel-based alloys, such as Alloy 706. Alloy 706 is a nickel-based superalloy used for high temperature applications in gas turbine engines. Such hold time fatigue cracking may severely reduce the service life of the gas turbine rotor disks.
The hold time fatigue cracking initiates at a surface of the part. Conventional remedies for reducing hold time fatigue cracking include applying compressive residual stresses to the surface using a suitable method such as shot peening. By providing surface compressive residual stresses, the net stress on the surface can be significantly reduced to prevent cracking. However, if the shot peening does not cover the entire surface, cracks may initiate at locations inadequately peened. In addition, any damage on the surface during handling can cause crack initiation at such locations. Once the crack is initiated microscopically, the crack may propagate rapidly and make the part unusable.
Another conventional remedy for reducing hold time fatigue cracking includes frequently inspecting the parts using non-destructive techniques. This approach requires the gas turbine to be shut down, cooled and partially dissembled to provide access to critical areas for inspection. The obvious disadvantage to this remedy is the required shut down time and the significant time loss. Further, detection of microscopic cracks by non-destructive methods is difficult and unreliable. Macroscopic or larger cracks may also be missed in locations difficult to access. The rapid propagation rate of these cracks may cause failure before the next scheduled inspection. One solution to this problem is to reduce the scheduled inspection intervals so cracks are more reliably detected. However, frequent shut down for inspection purposes is highly undesirable for power production turbines, for example. Another alternative approach is to estimate the life of each turbine part based upon material properties and/or exposure to stresses and high temperatures, and retire the part after a determined service time. This approach is currently applied to aircraft engine turbines, where some mandates are in place for how long a turbine part can be used. Such a solution applied to power generation turbines is not desirable because of the cost involved.